1. Field of Invention
The present invention relates to a single flare tube and to a joint structure of the single flare tube. The single flare tube includes a tubular portion and a flare portion attached to the front end of the tubular portion. The joint structure of the single flare tube includes the single flare tube, a flare nut and a union. The joint structure of the single flare tube can be used, for example, in a fluid circuit in order to connect one part of the fluid circuit to another part of the fluid circuit.
2. Description of Related Art
A joint structure of a single flare tube is well known among several kinds of joint structures of tubes. One example of a joint structure of a single flare tube is shown in FIG. 6. With reference to FIG. 6, a cylindrical hole 101a is drilled into a union 101, and an internal thread 101b is cut in a part of the cylindrical hole 101a. An external thread 102b is cut in an outer peripheral surface of a flare nut 102, and the flare nut 102 is connected with the union 101 by screwing together the external thread 102b and the internal thread 101b. The flare nut 102 has a longitudinal through-hole 102a. A single flare tube 103 has a longitudinal through-hole 105a for flowing fluid, and includes a flare 104 and a tube 105. The tube 105 passes through the through-hole 102a of the flare nut 102. The flare 104 is positioned at the front end part of the single flare tube 103, and an outer peripheral circumference of the flare 104 extends radially outward. The outer diameter of the flare 104 progressively decreases closer to the front end of the flare 104, and thus a front slanting surface 104b of the flare 104 is nearly slant-shaped.
The flare nut 102 with the single flare tube 103 positioned in the through-hole 102a is inserted into the cylindrical hole 101a. When the flare nut 102 is tightened at the thread 101b of the union 101 and the thread 102b of the flare nut 102, the flare 104 is located between a front surface 102c of the flare nut 102 and a slant shaped inner wall 101c of the union 101, and the single flare tube 103 is coupled to the union 101.
In the above-mentioned joint structure of the single flare tube, the conventional type flare 104 has a longitudinal section shaped like xcex9, and is elastically longitudinally deformed by tightening the threads of the flare nut 102 and the union 101. It had been thought that by this elastic deformation, the front surface 104b of the flare 104 could be tightly fitted to the interior wall 101c of the union 101.
In actual practice, however, the flare 104 can be plastically deformed by excessive tightening of the flare nut 102, as illustrated in FIG. 7. An inner circumferential groove 104a of the flare 104 then becomes out of shape, and not only the front end of the front surface 104b, but also the whole slanted, front surface 104b could contact the interior wall 101c of the union 101. In this case, since the contact area between the flare 104 and the interior wall 101c increases, the contact pressure between both parts is reduced. Then, the sealability of this area also decreases.
Furthermore, a phenomenon by which the flare 104 rotates together with the flare nut 102 (called dual-rotation) could happen in the condition where the flare 104 is sandwiched and pushed between the front end of the flare nut 102 and the inner wall 101c of the union 101. If the flare nut 102 continues to be tightened until the flare 104 becomes out of shape completely and the gap of the inner circumferential groove 104abecomes substantially zero (as shown in FIG. 7), a rotation angle of the flare 104 by this dual-rotation could become a non-negligible level. As a consequence, the single flare tube 103 could twist by an amount that can not be neglected.
In order to prevent the single flare tube 103 from rotating with the flare nut 102, low friction oil (for example, vegetable-based rust preventive oil) could be placed on the external thread 102b and the front surface 102c of the flare nut 102. In this case, however, an oil coating process is necessary which increases the cost. If the low friction oil accidentally happens to be placed on the front surface 104b, the friction between the front surface 104b and the inner wall 101c is lowered. Then, the flare 104 could be slippery to the inner wall 101c, which results in the dual-rotation of the flare 104 and the flare nut 102. Consequently, the twisting angle of the single flare tube 103 could become higher.
Furthermore, in the case where the union 101 is surface-coated, the inner wall 101c must be masked in order to avoid being surface-coated and to prevent the friction between the slanting surface 104b and the inner wall 101c from becoming lower, and to prevent the flare 104 from rotating with the flare nut 102. Since the masking process is necessary, it causes the cost to increase and the reliability to be lowered.
On the other hand, a curvature of an outer (radially outward facing) corner 103a located between the front end of the tube 105 and the rear end of the flare 104 is constant and high. In the case that the curvature of the outer corner 103a is high, a high tensile stress P is loaded to the surface of the outer corner 103a when the flare nut 102 is tightened firmly, and the tensile stress P on the outer corner 103a remains after fastening the single flare tube 103 to the union 101 with the nut 102. At the outer corner 103a the fatigue strength of the single flare tube 103 becomes lower when the tensile stress P remains.
In addition, if the curvature of the outer corner 103a of the single flare tube is high, tensile stress tends to concentrate on the outer corner 103a when an external force places a further load on the single flare tube 103.
As mentioned above, if the curvature of the outer corner 103a is high, the tensile stress P tends to remain and concentrate. Consequently, if a vibration adds to the single flare tube 103, for example, it is not easy to ensure the durability of the single flare tube 103 at the outer corner 103a. 
It is thus one object of the present invention to solve the aforementioned problem. Another object of the invention is to provide a single flare tube and a joint structure of the single flare tube in which a flare is not easily plastically deformed and a tensile stress does not easily remain.
According to one aspect of the invention, a joint structure of a single flare tube includes a single flare tube, a first connector member and a second connector member. The single flare tube includes a tube and a flare, and the flare is coupled with the tube at the front end of the tube. The flare extends radially outward relative to a longitudinal axis of the tube. The first connector member can be, for example, a flare nut that has a through-hole for receiving the tube of the single flare tube. The second connector member can be, for example, a union that is connectable with the first connector member. The flare nut and the union can have, for example, threads that engage each other. A width of a narrow inner circumferential groove of the outer radially extending part of the flare does not substantially change when the single flare tube is pushed and coupled with the union by coupling the flare nut with the union.
Since the width of the narrow inner circumferential groove formed inside of the flare is not substantially changed when the single flare tube is pushed against the union by the flare nut, according to the present invention, the flare does not deform plastically, and a contacting area between a front slanting surface of the flare and an inner wall formed inside of the union does not increase. Accordingly, the contacting pressure between the slanting surface of the flare and the inner wall of the union does not decrease after tightening the flare nut with the union.
Furthermore, according to another aspect of the present invention, the width of the narrow inner circumferential groove is substantially zero starting at a point that is radially closer to the longitudinal axis of the tube than is the external diameter of the tube. The narrow portion of the groove extends radially outward, and stays substantially zero when the single flare tube is pushed against and coupled with the union by connecting the flare nut with the union.
Since the single flare tube having the flare formed with the above-mentioned narrow inner circumferential groove is not easily deformed plastically, the flare does not deform plastically and a contacting area between a front slanting surface of the flare and an inner wall formed inside of the union does not increase. Accordingly, the contacting pressure between the slanting front surface of the flare and the inner wall of the union does not decrease after tightening the flare nut with the union.
Furthermore, according to another aspect of the present invention, a curvature at the (radially) outer side on the outer corner of the single flare tube is greater than a curvature at the (radially) inner side on the outer corner of the single flare tube on a longitudinal cut section. When a vibration is transmitted to the single flare tube by an external force, an outer corner between the front end of the tube and the rear end of the flare becomes deformed. As a consequence, a crack could occur on the corner if a high tensile stress remains on the corner. With this aspect of the present invention, however, since the curvature at the inner side on the outer corner of the single flare tube on the longitudinal section, where the greater stress is loaded by external force, is smaller, a tensile stress concentration is lower. Consequently, a crack does not occur easily on the single flare tube.